Organic light-emitting devices

ABSTRACT

An organic light-emitting device comprising a light-emissive organic layer interposed between first and second electrodes for injecting charge carriers into the light-emissive organic layer, at least one of said first and second electrodes comprising a plurality of layers including a first electrode layer having a high resistance adjacent the surface of the light-emissive organic layer remote from the other of the first and second electrodes, said first electrode layer comprising a high-resistance material selected from the group consisting of a mixture of a semiconductor material with an insulator material, a mixture of a semiconductor material with a conductor material and a mixture of an insulator material with a conductor material.

This application is a continuation of and claims priority to U.S. Ser.No. 09/868,351, filed Oct. 2, 2001 now U.S. Pat. No. 7,005,196, which isa 371 of PCT/GB99/04150, filed Dec. 15, 1999 and which claims priorityto Great Britain Application No. 9907120.1, filed Mar. 26, 1999; andGreat Britain Application No. 9827699.1, filed Dec. 16, 1998, all ofwhich are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to organic light-emitting devices (OLEDs) and amethod for improving the uniformity of current density of OLEDs having alight-emissive organic layer containing intrinsic defects.

The present invention also relates to organic light-emissive deviceshaving patterned electrodes.

BACKGROUND TO THE INVENTION

Organic light-emitting devices such as described in U.S. Pat. No.5,247,190 or in U.S. Pat. No. 4,539,507, the contents of which areincorporated herein by reference, have great potential for use invarious display applications. According to one method, an OLED isfabricated by coating a glass or plastic substrate with a transparentfirst electrode (anode) such as indium tin oxide (ITO). At least onelayer of a thin film of an electroluminescent organic material is thendeposited prior to a final layer which is a film of a second electrode(cathode) which is typically a metal or alloy.

In many practical applications, the layer of electroluminescent organicmaterial has a thickness of the order of 100 nm in order to ensure apractical operating voltage. It is typically deposited on the firstelectrode by a spin-coating technique. If the organic material iscontaminated with particles having a size of the order of the thicknessof the organic layer, not only will these particles themselves give riseto defects in the resulting organic layer, their presence disrupts themovement of the fluid organic material over the surface of the firstelectrode layer leading to variations in the thickness of the resultingorganic layer about the particle, and in the worst case leading to theformation of holes in the organic layer through which the underlyinglayer (electrode layer) is exposed.

Defects in the organic layer can also be caused by, for example,inherently poor film-forming properties of the organic material, or byphysical damage to the organic layer after deposition.

A typical defect site is shown in FIG. 3. The electroluminescent organiclayer 106 has been deposited by spin coating on a glass substrate 102coated with an indium tin oxide (ITO) anode layer. The existence of alarge particle 107 has led to a defect site 109 comprising the particle107 itself and a pinhole 111. A cathode layer 110 is formed over theelectroluminescent organic layer 106.

Localized defects of the kind shown in FIG. 3 can manifest themselvesduring device operation as a current anomaly (short) where a largeproportion of the current becomes localized in the area of the defect.This leads, inter alia, to problems of device reproducibility and is aparticular problem in dot matrix devices since it provides alternativecurrent paths that lead to the wrong pixels being lit.

In order to prevent these kind of defects, the deposition of the organiclayer is typically carried out in a clean room with a view to preventingcontamination and typically involves filtering the organic materialprior to spinning to remove large particles therefrom. However, atypical clean room has particle size levels specified down to 300 nm andthe organic material is only typically filtered to about 450 nm, sincethe elimination of particles having smaller sizes requires greatexpense.

The light-emissive organic material will therefore often still becontaminated with particles having a size of the order of the thicknessof the organic layer to be deposited, which will, as mentioned above,lead to defects in the resulting organic layer. Furthermore, even if thecontamination by such large particles could be completely eliminated,defects can still arise during the manufacturing process as a result,for example, of inherently poor film-forming properties of the organicmaterial itself, or due to physical damage inadvertently inflicted onthe organic layer after deposition.

One known technique of removing the defect particles after production ofthe device is by passing a very high current through the device to“burn-out” the defect particles by vaporizing them. However, thistechnique is not applicable to all defect particles and cannot be usedto resolve the problem of large shorts. Moreover, it does notnecessarily deal with problems that may manifest themselves in thelifetime of the device. It is therefore an aim of the present inventionto reduce the problem of current anomalies in an organic light-emittingdevice.

In organic light-emissive devices (OLED's) such as those described inour earlier U.S. Pat. No. 5,247,190 or in Van Slyke et al.'s U.S. Pat.No. 4,539,507, light emission from the at least one organic layer occursonly where the cathode and the anode overlap and therefore pixelationand patterning is achieved simply by patterning the electrodes. Highresolution is readily achieved and is principally limited only by theoverlap area of the cathode and the anode and thus by the size of thecathode and the anode. Dot-matrix displays are commonly fabricated byarranging the cathode and the anode as perpendicular arrays of rows andcolumns, with the at least one organic layer being disposedtherebetween.

Low resolution dot-matrix displays can, for example, be fabricated bycoating at least one organic electroluminescent layer onto a substratehaving thereon an array of indium-tin oxide (ITO) lines which act as ananode. A cathode comprising an array of lines perpendicular to those ofthe anode is provided on the other side of the at least one organiclayer. These cathode lines may, for example, be lines of aluminum or analuminum-based alloy which can be evaporated or sputtered through aphysical shadow mask. However, shadow masking may not be desirable forvarious reasons. In particular, there are significant constraints on theuse of shadow masks when displays of large area and/or high resolutionare required. In order to produce such electrode line arrays and otherpatterns of large area and/or high resolution one would normally have touse various forms of lithography.

In order to fabricate efficient and stable OLED's with the desiredelectrical and light output characteristics great care must normally betaken in the design and construction of the interfaces between anyorganic layer and the electrodes. The particular importance of theseinterfaces is due to the fact that charge carriers should be injectedefficiently from the electrodes into the at least one organic layer.

Maintaining the desired electrical and light output characteristic ofthe pixels in an OLED display when lithographic processes are used tofabricate the electrode patterns, in particular where those patterns areon top of the at least one organic layer, is not trivial owing to therisk of the lithographic processes modifying and potentially damagingthe organic layer/electrode interfaces and the vicinity. Such damageduring lithography may originate from the photoresist, the developers,the etching processes (both dry and wet, negative and positivetechniques and etch and lift-off) or the solvents used. It should bementioned here that conjugated polymers are often deposited from and aregenerally soluble in organic solvents.

Plasma etching/ashing is very often used in lithography to remove thephotoresist or residual photoresist which may not have been washed offthe developer. Organic electroluminescent and charge transportingmaterials would normally be damaged, modified and/or etched very rapidlyin such dry etching/ashing processes if directly exposed to the plasma.

One method of protecting the organic electroluminescent and chargetransporting materials from the effects of the electrode patterningprocesses is disclosed in WO97/42666 in which a thin barrier layercomposed of a dielectric material is interposed between the conductiveelectrode layer and the layer of light-emissive organic material.

The inventors of the present invention have identified the requirementfor an improved construction which allows for the use of variouslithographic processes to form the electrode on top of at least oneorganic layer without significantly changing the electrical and lightoutput characteristics of the display, and which meets todays demandsfor increased efficiency, reliability and durability. It is thereforeanother aim of the present invention to provide a device which meetsthese requirements.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan organic light-emitting device comprising a light-emissive organiclayer interposed between first and second electrodes for injectingcharge carriers into the light-emissive organic layer and means forlimiting the current flow through any conductive defect in saidlight-emissive organic layer. In contrast to the “burn out” techniquereferred to above, the incorporation into the device of means forlimiting the current flow through any conductive defect in thelight-emissive layer prevents any current anomalies arising during thelifetime of the device from rising to such a level as to significantlyaffect device operation in the manner described above.

Preferably the means are incorporated into at least one of said firstand second electrodes of the device. In particular, the electrode maycomprise a plurality of layers including a first electrode layeradjacent the surface of the light-emissive organic layer remote from theother of the first and second electrodes and having a resistanceselected such that it is not too high to cause a significant increase inthe drive voltage of the device, yet high enough to prevent excessivecurrents at any conductive defect in said light-emissive organic layer.

According to one embodiment of the invention, the first electrode layermay comprise a high-resistance material selected from the groupconsisting of a mixture of a semiconductor material with an insulatormaterial, a mixture of a semiconductor material with a conductormaterial and a mixture of an insulator material with a conductormaterial. The use of a layer of the above-mentioned mixtures ofmaterials as the high resistance electrode layer has the advantage thatthe resistance of the high resistance electrode layer can be easilyadjusted to the desired value by simply adjusting the relativeproportions of the components of the mixture accordingly.

In the case of a cathode, the first electrode layer preferably comprisesat least one material having a low work function, preferably less than3.7 eV, and further preferably less than 3.2 eV, to improve theelectron-injecting performance of the cathode.

According to a second aspect of the present invention, there is providedan organic light-emitting device comprising a light-emissive organiclayer interposed between first and second electrodes for injectingcharge carriers into the light-emissive organic layer and means forelectrically isolating any conducting defect in the organic layer froman associated electrode. Any current anomalies arising during thelifetime of the device according to this aspect of the invention areshort-lived—the conducting defect in the organic layer giving rise tothe current anomaly is rapidly isolated from the associated electrode bymeans incorporated in the device.

These means are preferably incorporated into at least one of said firstand second electrodes, which may comprise a plurality of layersincluding a thin first electrode layer adjacent the surface of thelight-emissive organic layer remote from the other of the first andsecond electrodes, the dimensions and material properties of said thinfirst electrode layer being chosen such that, adjacent a conductingdefect in said organic layer, said layer will vaporize when subject toan anomalous current resulting from said conducting defect.

According to one embodiment of the invention, the electrode is opaqueand comprises a plurality of layers including a thin first electrodelayer comprising a low work function material adjacent the surface ofthe light-emissive organic layer remote from the other of the first andsecond electrodes, and a second electrode layer adjacent the surface ofthe first electrode layer remote from the light-emissive organic layer,said second electrode layer comprising a layer of a high-resistancematerial selected from the group consisting of a semiconductor material,a mixture of a semiconductor material with an insulator material, amixture of a semiconductor material with a conductor material and amixture of an insulator material and a conductor material.

Alternatively, the electrode may have a first electrode layer comprisinga plurality of sub-electrodes, each sub-electrode being connected toeach of any sub-electrodes directly surrounding it via a fusible link,each fusible link adapted to break when subject to a current exceeding aspecified value to electrically isolate the respective sub-electrodefrom the other sub-electrodes.

The thin first electrode layer in this second aspect of the presentinvention preferably has a thickness in the range of 0.5 to 10 nm, andis further preferably 5 nm or less.

According to a third aspect of the present invention, there is providedan organic light-emitting device comprising a light-emissive organiclayer interposed between first and second electrodes for injectingcharge carriers into the light-emissive organic layer, at least one ofsaid first and second electrodes comprising a plurality of layersincluding a first electrode layer having a high resistance, said firstelectrode layer having a thickness greater than the light-emissiveorganic layer, such that any intrinsic defects in the light-emissiveorganic layer are covered by the first electrode layer.

According to one embodiment, the first electrode layer is disposedadjacent the surface of the light-emissive organic layer remote from theother of the first and second electrodes.

By making the thickness of the high resistance first electrode layergreater than that of the light-emissive organic layer, any pinholedefects in the light-emissive organic layer are completely filled makingit possible to further ensure that there are no areas of thelight-emissive organic layer left exposed to make direct contact with anoverlying conductive layer.

The high resistance layer in this third aspect of the inventionpreferably comprises a semiconductor material, a mixture of asemiconductor material with a conductor material, a mixture of asemiconductor material with an insulator material or a mixture of aconductor material with an insulator material.

According to a fourth aspect of the present invention, there is provideda method for improving the uniformity of current density of an organiclight-emitting device comprising a light-emissive organic layerinterposed between first and second electrodes for injecting chargecarriers into the light-emissive organic layer, the method comprisingthe step of forming one of the first and second electrodes from aplurality of layers including a first electrode layer having a highresistance comprising a semiconductor material, a mixture of aninsulator material with a semiconductor material, a mixture of aninsulator material with a conductor material, or a mixture of asemiconductor material with a conductor material.

In each of the above aspects of the invention, the high-resistanceelectrode layer is preferably capped with a layer of a conductormaterial such as a layer of aluminum.

The resistance of the high resistance electrode layer in the first tofourth aspects of the present invention is preferably selected such thatit is not too high to cause a significant increase in the drive voltage(since this will reduce the power efficiency of the device) but is highenough to prevent excessive currents at defect sites. Typically for anelectrode layer of thickness lying in the range of 100-10000 nm, theresistivity lies in the range 1 to 10⁵ Ωcm.

Suitable semiconductor materials for use in the above aspects of thepresent invention include, for example, Ge, Si, α-Sn, Se, ZnSe, ZnS,GaAs, GaP, CdS, CdSe, MnS, MnSe, PbS, ZnO, SnO, TiO₂, MnO₂ and SiC.

Suitable insulator materials for use in the above aspects of the presentinvention include, for example, insulating oxides, nitrides andfluorides such as Al₂O₃, SiO₂, LiO, AIN, SiN, LiF and CsF. Suitableconductor materials for use in the present invention include, forexample, metals such as Al and Ag.

Suitable low work function materials for use in the present inventioninclude, for example, Ca, Li, Yb, LiF, CsF and LiO.

The use of the cathode to combat the undesirable effects of intrinsicdefects in the light-emissive organic layer is particularly advantageouswhen the cathode is deposited in a vacuum because of the ability to keepparticulate levels extremely low.

According to a sixth aspect of the present invention, there is provideda light-emissive device comprising a layer of light-emissive materialarranged between first and second electrode layers such that chargecarriers can move between the first and second electrode layers and thelight-emissive material, wherein at least the first electrode layercomprises a plurality of sub-electrodes, each sub-electrode beingconnected to each of any sub-electrodes directly surrounding it via afusible link, each fusible link adapted to break when subject to acurrent exceeding a specified value to electrically isolate therespective sub-electrode from the other sub-electrodes.

In a preferred embodiment of the sixth aspect of the present invention,the plurality of sub-electrodes are arranged to create an ordered arrayof parallel rows and columns, and each of the sub-electrodes isconnected via a fusible link to each of any sub-electrodes directlyadjacent to it in the same column and row.

The size and spacing of the sub-electrodes is preferably selected suchthat, during operation of the device, the light emitted by thelight-emissive device appears to the human eye to be continuous inintensity across the whole area of light emission.

According to a seventh aspect of the present invention there is providedan organic light-emissive device comprising a light-emissive organicregion interposed between first and second electrodes for injectingcharge carriers into the light-emissive organic region, at least one ofsaid first and second electrodes comprising: a high-resistance firstelectrode layer adjacent the surface of the light-emissive organicregion remote from the other of the first and second electrodes, saidfirst electrode layer covering substantially the entire area of thesurface of the light-emissive organic region remote from the other ofthe first and second electrodes and comprising a high-resistancematerial selected from the group consisting of a mixture of asemiconductor material with an insulator material, a mixture of asemiconductor material with a conductor material and a mixture of aninsulator material with a conductor material; and a patterned conductivesecond electrode layer adjacent the surface of the first electrode layerremote from the light-emissive organic region.

In the seventh aspect of the present invention, the first electrodelayer is, as described above, formed over substantially the entiresurface of the light-emissive organic region. In other words, the firstelectrode layer is formed over at least that area of the surface of thelight-emissive organic region corresponding to the area occupied by thesecond electrode layer as defined by the laterally outermost edges ofthe patterned second electrode layer.

The term “patterned electrode layer” refers to a plurality of electrodeelements which are only connected via the underlying high resistancefirst electrode layer. The patterned electrode layer preferablycomprises an ordered array of separate elements such as a series ofparallel rows or columns which are only connected via the underlyinghigh resistance first electrode layer.

The resistance of the high resistance first electrode layer in thisseventh aspect of the present invention is determined such that it issufficiently high to prevent significant current leakage betweenelements of the patterned second electrode layer, but is not so high asto significantly increase the voltage required to operate the device.

The use of a material comprising a physical blend of an insulatormaterial and a semiconductor material, or a physical blend of asemiconductor material and a conductor material or a physical blend of aconductor material and an insulator material as the barrier layer hasthe significant advantage that the resistivity of the layer can bereadily adjusted in accordance with the requirements of the individualdevice by appropriately varying the relative proportion of each materialin the blend.

The high resistance first electrode layer of this seventh aspect of thepresent invention is preferably composed of a physical blend of aconductor and an insulator or semiconductor, preferably a physical blendof a conductor and an insulator, since the increased conductivity of theblend realized by the inclusion of a conductor material means that thethickness of the high-resistance first electrode layer can be increasedwithout causing a significant increase in the voltage required tooperate the device. This ability to substantially increase the thicknessnot only provides the possibility to enhance the protection of theunderlying organic layer or layers from the effects of etching/ashingprocesses but also provides the means to compensate for the adverseeffects of any defects (such as particles of contamination or pinholes)which inevitably exist in the underlying organic film even with the highdegree of cleanliness provided by the modern clean room. For example,covering any such defects substantially reduces the existence ofundesirable low-resistance pathways within the device, thereby improvingthe performance of the device. A barrier layer of increased thicknessalso provides increased protection of the underlying organic layeragainst the ingress of reactive ambient species such as moisture andoxygen which can react with the organic material resulting in blackspots.

Suitable semiconductor materials include, but are not limited to, Ge,Si, α-Sn, Se, ZnSe, ZnS, GaAs, GaP, CdS, CdSe, MnS, MnSe, PbS, ZnO, SnO,TiO₂, TiO₂, MnO₂ and SiC.

Suitable insulator materials include, but are not limited to, oxides,nitrides and halides such as fluorides. The insulator material ispreferably selected from the group consisting of Al₂O₃, SiO₂, LiO, AIN,SiN, LiF and CsF.

Suitable conductor materials include, but are not limited to, metals,preferably Al or Ag.

According to an embodiment of the seventh aspect of the presentinvention, the first electrode layer forms the cathode of the device andcomprises at least one material comprising an element having a low workfunction (preferably 3.7 eV or less, and further preferably 3.0 eV orless) such as Li, Ca or Cs whereby the electron injecting performance ofthe electrode is enhanced. Electrode layers comprising a materialincluding Li or Ca are particularly preferred. The first electrode layeris preferably comprised of a mixture selected from the group consistingof LiF/Al, Ca/Ge, Li/Si, Ca/ZnO, LiF/ZnSe and CsF/ZnS.

According to an alternative embodiment of the seventh aspect of thepresent invention, the first electrode layer forms the anode of thedevice and comprises at least one material including an element having ahigh work function (preferably greater than 4.5 eV and furtherpreferably greater than 5.0 eV) whereby the hole injecting performanceof the electrode is enhanced. In this alternative embodiment, it ispreferred that the first electrode layer comprises a material selectedfrom the group consisting of Au, Pd, Ag and indium-tin oxide (ITO).

The first electrode layer preferably has a thickness in the range of 0.5to 1.0 microns, and is composed of a material having a resistivity, ρ inthe range of 10² to 10⁵ Ωcm.

According to an eighth aspect of the present invention, there isprovided an organic light-emissive device comprising a light-emissiveorganic region interposed between first and second electrodes forinjecting charge carriers into the light-emissive organic region, atleast one of said first and second electrodes comprising a plurality oflayers including a high-resistance first electrode layer adjacent thesurface of the light-emissive organic region remote from the other ofthe first and second electrodes, said first electrode layer formed oversubstantially the entire area of the surface of the light-emissiveorganic region remote from the other of the first and second electrodes,and having a thickness greater than the light-emissive organic regionwhereby adverse effects of any defects in the light-emissive organicregion are compensated for by the first electrode layer; and a secondelectrode layer adjacent the surface of the first electrode layer remotefrom the light-emissive organic region, said second electrode layercomprising a patterned conductive layer.

In this eighth aspect of the present invention, the thickness of thefirst electrode layer is preferably in the range of 0.5 to 1 micron; andthe first electrode layer preferably comprises a material selected fromthe group consisting of a semiconductor material, a mixture of asemiconductor material and an insulator, a mixture of a semiconductormaterial and a conductor material and a mixture of an insulator materialand a conductor material.

According to a ninth aspect of the present invention, there is provideda method of forming an electrode of an organic light-emissive devicecomprising a light-emissive organic region interposed between first andsecond electrodes for injecting charge carriers into the light-emissiveorganic region, the method comprising forming one of the first andsecond electrodes by first forming a high-resistance first electrodelayer over substantially the entire area of the surface of thelight-emissive organic region remote from the other of the first andsecond electrodes, said first electrode layer comprising a materialselected from the group consisting of a semiconductor material, amixture of a semiconductor material with an insulator, a mixture of asemiconductor material with a conductor material and a mixture of aninsulator material with a conductor material; and then forming a secondelectrode layer on the surface of said first electrode layer remote fromthe light-emissive organic region, said second electrode layercomprising a patterned conductive layer.

According to a tenth aspect of the present invention, there is providedan organic light-emissive device comprising a light-emissive organicregion interposed between first and second electrodes for injectingcharge carriers into the light-emissive organic region, at least one ofsaid first and second electrodes comprising: a first electrode layercomprising an insulator material adjacent the surface of thelight-emissive organic region remote from the other of the first andsecond electrodes; a high-resistance second electrode layer adjacent thesurface of the first electrode layer remote from the light-emissiveorganic region; and a patterned conductive third electrode layeradjacent the surface of said second electrode layer remote from thefirst electrode layer; wherein said first and second electrode layerscover substantially the entire area of the surface of the light-emissiveorganic region remote from the other of the first and second electrodes;and said second electrode layer comprises a high-resistance materialselected from the group consisting of a semiconductor material, amixture of a semiconductor material with an insulator material, amixture of a semiconductor material with a conductor material and amixture of an insulator material with a conductor material.

In this tenth aspect of the present invention, the term “patternedelectrode layer” refers to a plurality of electrode elements which areonly connected to each other via the underlying electrode layers, andare preferably only connected to each other via the underlying highresistance second electrode layer.

The provision of a thin layer of an insulator material adjacent to theorganic light-emissive region as well as an overlying high-resistanceelectrode layer has the following additional advantage. The chargecarrier injecting performance can be further enhanced at the interfacebetween the electrode and the light-emissive organic region by using amaterial containing a low work function element in the case of a cathodeor a material containing a high work function element in the case of ananode without significantly increasing the operating voltage of thedevice and detracting from the function of the first and secondelectrode layers as a whole which is to prevent lateral current leakage(cross-talk) as well as protecting the underlying organic region.

In this tenth aspect of the present invention, the first electrode layerpreferably comprises a layer of a dielectric oxide, nitride or halidesuch as a fluoride. Particularly preferred materials for use in the caseof cathodes are LiO, LiF and CsF.

In each of the seventh to tenth aspects of the present invention, thelight-emissive organic region may, for example, be composed of a singlelayer of a light-emissive organic material such as a light-emissivepolymer, or it may include one or more additional organic layers whichmay function as additional light-emissive layers or as charge injectionand/or transport layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:—

FIG. 1 is a cross-sectional view of an OLED according to an embodimentof the present invention;

FIG. 2 is a cross-sectional view of an OLED for explaining the principleof one aspect of the present invention;

FIG. 3 is a schematic cross-sectional view of an OLED having a typicaldefect site caused by particulate contamination of the organic materialduring spin-coating;

FIG. 4 is a cross-sectional view of an OLED according to anotherembodiment of the present invention;

FIG. 5 shows a schematic view of a light-emissive device according toanother embodiment of the present invention;

FIG. 6 is a schematic plan view of a section of the anode layer of thedevice shown in FIG. 5;

FIG. 7 is a cross-sectional view of a device according to anotherembodiment of the present invention; and

FIG. 8 is a cross-sectional view of a device according to anotherembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an OLED according to a first embodiment of the presentinvention. A glass substrate 2 having a thickness of 1.1 mm is coatedwith a layer 4 of indium tin oxide (ITO) with a sheet resistance of 15Ohms/sq. to a thickness of 150 nm. Although not shown in FIG. 1, this ispatterned to form a series of parallel strips using, for example,standard photolithographic and etch processes. A layer 6 ofpolyethylenedioxythiophene doped with polystyrene sulphonic acid(PEDT:PSS) is spun on the anode layer 4 and subsequently baked at 150°C. to remove water leaving a layer of 50 nm thickness. A layer 8 of alight-emissive polymer such as a blend of 5%poly(2,7-(9,9,di-n-octylfluorene)-3,6-(benzothiadiazole) with 95%poly(2,7(9,9-di-n-octylfluorene) (5BTF8) doped withpoly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene))(TFB) is then spun on to the layer 6 of PEDT:PSS to a thickness of 75nm. A cathode layer 10 is then formed on the layer 8 of light-emissivepolymer.

A standard vacuum thermal evaporation technique is used to deposit thecathode layer in view of the fact that, being a relatively low-energytechnique, it causes minimal damage to the underlying layer oflight-emissive polymer. If the possibility of damage to the underlyingorganic layer is not a concern, sputtering is a desirable techniquebecause it is a conformal deposition technique. In the case ofsputtering, neon is preferably used as the discharge gas.

In this case, the cathode layer 10 is a layer of LiF co-evaporated withAl. This cathode layer 10 is deposited to a thickness of between 0.5 and1 micron to ensure that the entire surface of the underlying organiclayer, and hence any defects therein, is covered by the cathode layer. Alayer 12 of aluminum is deposited on top of this layer to a thickness of0.5 microns. This top layer of aluminum 12 can, for example, bedeposited by evaporation. Although not shown in FIG. 1, the cathodecomprising the LiF—Al cathode layer 10 and the aluminum top layer 12 isalso patterned in the form of a series of parallel strips running in adirection orthogonal to the series of parallel anode strips, whereby anordered array of pixels is formed defined by the points at which eachseries of cathode and anode strips overlap.

The LiF has a dual function. It is a low work function material andtherefore assists the injection of electrons into the light-emissiveorganic layer. It is also an insulator resulting in a layer having ahigh resistance.

The high-resistance LiF/Al layer conducts via a percolation mechanism.

The relative proportions of LiF and Al in the LiF—Al layer 10 will bedetermined according to the desired resistivity. The desired resistivityis itself determined according to the number and area of the defectsexisting in the underlying light-emissive organic layer 8. A method fordetermining a suitable resistivity is described below with reference toFIG. 2 which shows an OLED comprising a light-emissive organic layer 18containing a plurality of pinhole defects 30 which are the major causeof current anomalies in OLEDs.

The light-emissive organic layer 18 is sandwiched between a firstcathode layer 20 and an ITO anode layer 14 coated on a glass substrate12. The first cathode layer is coated with a layer of aluminum 22.

It is supposed, by way of example, that the current density (j) of thedevice at a typical operating voltage of 3V would be 1 mA/cm² if thelight-emissive organic layer 18 did not contain any pinhole defects.

It is desired that the current density attributable to the existence ofthe pinhole defects represents only a small proportion of the currentdensity that would be observed if there were no pinholes existing in thelight-emissive organic layer. For example, it is preferred that thefirst cathode layer is of a sufficiently high resistance that thecurrent density attributable to the defects is at most 1% of the currentdensity that would be observed if there were no pinholes existing in thelight-emissive organic layer.

The current density through the pinhole defects can be calculated to be:j _((def)) =NVA/ρtwhere N is the density of defects (per unit area); A is the average areaof each defect; V is the operating voltage; ρ is the resistivity of thecathode layer; and t is the thickness of the first cathode layer 20.

Let us now suppose that the thickness of the first cathode layer 20 is0.5 microns, and that there are 100 defects each of area 1 μm².

Then, at the operating voltage of 3V mentioned above, the currentdensity attributable to the defects would be approximately 60/ρ mA/cm².

In order for this current density to represent 1% or less of the currentdensity that would be observed if there were no pinhole defects (whichis supposed as above to be 1 mA/cm²), the resistivity of the material ofthe first cathode layer would have to be about 6000 Ωcm or greater.

The voltage drop across a first cathode layer having a thickness of 0.5microns and composed of material having a resistivity of 6000 Ωcm wouldonly be about 0.3 mV when the current density is 1 mA/cm². This layerwill therefore have a negligible effect on the power efficiency whilstimproving the uniformity of the current density of the OLED inoperation.

The existence of particle defects in the light-emissive organic layerhave been ignored on the basis that their effect is negligible comparedto that of the pinhole defects. However, if the effects of any suchdefects are not negligible, it will be clear to the skilled person inlight of the above how to take the effect of such particle defects intoconsideration when determining a suitable resistivity for the highresistance cathode layer.

Hereunder is provided a method for calculating the optimum value ofresistivity for the high resistance layer for a device of the kind shownin FIG. 1 with defective areas that would, without this high resistancelayer, allow direct connection between the low resistance cathode andanode. The film is optimized for maximum efficiency.

For a device with no defects operating at a current density ofI₀(mA/cm²) and voltage V₀ (Volts) with luminosity L₀(Cd/m²) has aluminous efficiency η₀(1m/W) of

$\begin{matrix}{\eta_{0} = {\frac{\pi\; L_{0}}{10\; V_{0}I_{0}}.}} & (1)\end{matrix}$

If we now introduce defects into the device where the defective area asa ratio of the total area is D and the areal resistance of these defectsis R_(D)(kΩ cm²), the average current density through the whole deviceat the same voltage is

$\begin{matrix}{I = {{\left( {1 - D} \right)I_{0}} + {D{\frac{V}{R_{D}}.}}}} & (2)\end{matrix}$

If we assume that the defective areas do not emit any light then thelight emitted by the defective light-emissive organic layer is justgiven byL=(1−D)L ₀.  (3)

If we now introduce a high resistance cathode layer with arealresistivity R_(H)(kΩcm²) then to get I₀ flowing through thenon-defective areas, the voltage across the device needs to be increasedtoV=V ₀ +I ₀ R _(H).  (4)

The average current density flowing through our device is from equation2

$\begin{matrix}{I = {{\left( {1 - D} \right)I_{0}} + {D{\frac{V}{R_{D} + R_{H}}.}}}} & (5)\end{matrix}$

The new efficiency η is then given by combining equations 3, 4 and 5 togive

$\begin{matrix}{\eta = {\frac{{\pi\left( {1 - D} \right)}\left( {R_{D} + R_{H}} \right)L_{0}}{10\left( {V_{0} + {I_{0}R_{H}}} \right)\left\{ {{\left( {1 - D} \right)\left( {R_{D} + R_{H}} \right)I_{0}} + {D\left( {V_{0} + {I_{0}R_{H}}} \right)}} \right\}}.}} & (6)\end{matrix}$

In general, if the defective areas are a problem then they will have avery low resistance compared to the high resistance layer, i.e.R_(D)<<R_(H).  (7)

The efficiency then becomes

$\begin{matrix}{\eta = {\frac{{\pi\left( {1 - D} \right)}R_{H}L_{0}}{10\left( {V_{0} + {I_{0}R_{H}}} \right)\left\{ {{\left( {1 - D} \right)R_{H}I_{0}} + {D\left( {V_{0} + {I_{0}R_{H}}} \right)}} \right\}}.}} & (8)\end{matrix}$

If we differentiate this with respect to R_(H) to find the maximumefficiency we find that

$\begin{matrix}{R_{H}^{MAX} = {\frac{\sqrt{D}V_{0}}{I_{0}}.}} & (9)\end{matrix}$

The value of the high resistance layer that maximizes the efficiency ata particular operating point (determined by I₀ and V₀) depends on thesquare root of the fractional defective area.

The optimum resistivity of the high resistance layer will depend on thethickness of the high resistance layer which is in turn determinedaccording to the size and shape of the defect causing the short and themethod of deposition of the high resistance layer. If the method ofdeposition is one which covers all surfaces conformally then the highresistance layer can be any thickness. If however the method ofdeposition is a line of sight method such as evaporation from a fixedsource to a fixed target then the thickness has to be, in general,greater than the height of the defect. If the thickness of the highresistance layer is taken to be t_(H) (in cm) and the optimumresistivity ρ_(H) then

$\begin{matrix}{\rho_{H} = {\frac{R_{H}^{MAX}}{t_{H}}.}} & (10)\end{matrix}$

It is thus clear that the optimum values of thickness and resistivity ofthe high resistance layer depend on the size of the defective area, thenature of the defect, the deposition method and the operating point ofthe device.

FIG. 4 is a cross-sectional view of an organic light-emitting deviceaccording to another embodiment of the present invention. The substrate202, anode layer 204, organic layers 206, 208 are identical to those ofthe first embodiment described above. A thin layer of calcium 209 havinga thickness of 5 nm is formed on the surface of the organic layer 208.This layer 209 is preferably formed by vacuum evaporation. A layer ofsilicon 210 having a thickness of 0.5 microns is formed on the thinlayer of calcium 209 as a high-resistance layer, and a layer of aluminum212 having a thickness of 0.5 microns is formed on top of the layer ofsilicon 210.

The use of a thin layer of a conductor material (in this case, calcium)between the high-resistance layer and the light-emissive organic layeris advantageous as it effectively acts as a fuse. If a portion of thethin conductor layer is subject to an anomalously high current as resultof a defect in the portion of the organic layer underlying that portionof the thin conductor layer, that portion of the thin conductor layervaporizes thereby stopping current flowing through the conducting defectand improving the performance of the device. The conducting defects canbe isolated in this way by passing a high current through the deviceafter production is completed.

Although the embodiments described above are devices having ahigh-resistance cathode, alternatively a high-resistance anode can beemployed in the case, for example, that an OLED is produced by firstforming a cathode on a glass substrate, depositing a layer oflight-emissive organic material on the cathode by spinning, and finallyforming an anode on the light-emissive organic layer. In the case of ananode, it is preferred that the high-resistance electrode layercomprises a high work function material, or that a thin layer of a highwork function material is interposed between the high-resistanceelectrode layer and the light-emissive organic layer.

With reference to FIG. 5, there is shown a light-emissive deviceaccording to the sixth aspect of the present invention for use in thelight-emitting display according to the fifth aspect of the presentinvention. This device is intended for use as a backlight. It comprisesa glass substrate 302, an anode layer 304 deposited on the glasssubstrate 302, an organic hole transport layer 306 deposited on theanode layer 304, an electroluminescent polymer layer 308 deposited onthe hole transport layer 306, and a continuous metallic cathode layer310 deposited on the electroluminescent polymer layer 308. FIG. 6 showsa schematic plan view of a section of the anode layer as deposited onthe glass substrate to illustrate the nature of the patterning of theanode layer 304. It comprises an ordered two-dimensional array of smallsub-electrodes 320 arranged to form an array of parallel rows andcolumns. Each of the co-planar sub-electrodes is formed under adifferent portion of the hole transport layer 306. The dimension of thearea of these sub-electrodes and the spacing between them is made smallenough that a viewer of the light produced by the device cannot detectthem under normal viewing conditions. Each of the sub-electrodes 320 isconnected to those sub-electrodes directly adjacent to it in the samerow and column by a fusible link 322. The material and dimensions ofeach fusible link are selected such that under normal operatingconditions very little voltage is dropped across the fusible link, butsuch that, if subject to an anomalously high current (caused, forexample, by a defect in the portion of the organic layers situatedbetween the cathode and the sub-electrode), it will overheat and blowthereby isolating the defective site from the rest of the backlight,with a resulting improvement in the performance of the device.

The sub-electrodes of the anode and the fusible links can, for example,be made of indium-tin oxide (ITO). The patterned array formed by thesub-electrodes and fusible links can, for example, be formed by firstdepositing a continuous layer of ITO on the glass substrate and thenselectively etching the continuous layer using for example, aphotolithographical technique, to form the patterned array.Alternatively, the sub-electrodes and the fusible links may be made ofdifferent materials.

The cathode may additionally or alternatively be formed ofsub-electrodes connected by fusible links in the same manner asdescribed above for the anode. However, in the type of device describedabove in which the cathode layer is deposited on top of the relativelysensitive organic layers, care normally has to be taken not to causeundue damage to the underlying organic layers. For this reason, it ispreferable that the patterned cathode layer be formed by depositionthrough a shadow mask rather than by an etching technique.

Another embodiment of a light-emissive organic device according to thepresent invention is shown in FIG. 7. In this embodiment, a glasssubstrate 402 of thickness 1.1 mm is coated with indium tin oxide (ITO)404, which has a sheet resistance of 15 Ohms/sq., to a thickness of 150nm. This coating 404 of ITO is patterned to form a series of parallelrows using standard photolithographic and etch processes. A layer 406 ofpolyethylenedioxythiophene doped with polystyrene sulphonic acid(PEDT/PSS) is then formed on the ITO/glass substrate by spin-coating andbaked at 150° C. to remove water leaving a layer 406 having a thicknessof 50 nm. A layer 408 of light-emissive polymer is then deposited ontothe layer 406 of PEDT/PSS also by spin coating. This layer could be alayer of a blend of 5% ofpoly(2,7-(9,9-di-n-octylfluorene)-3,6-(benzothiadiazole) and 95% ofpoly(2,7-(9,9-di-n-octylfluorene) (5BTF8) doped withpoly(2,7-(9,9-di-n-octylfluorene-(1,4-phenylene-((1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene))(TFB) and has a thickness of 75 nm. A layer 410 of a LiF/Al blend isthen deposited on to the layer 408 of light-emissive polymer by theco-evaporation of LiF with Al in a vacuum chamber to form an ohmiccontact on the light-emissive polymer layer 408. The LiF/Al blend layer410 is deposited to a thickness sufficient to cover any defects on thesurface of the light-emissive polymer layer 408. In the case that thedevice is prepared in a class 100 clean room, the thickness would bebetween 0.5 and 1 micron. An aluminum layer 412 is then deposited overthe layer 410 of LiF/Al to a thickness of 0.5 microns, and is patternedusing conventional photolithographic techniques to form a series ofregularly spaced parallel columns running in a direction orthogonal tothe series of parallel rows of ITO to thereby define a regular matrix ofpixels where the series of ITO rows and Al columns spatially overlapwith each other.

The LiF/Al physical blend is an isotropic conductor which conducts via apercolation mechanism wherein the resistivity of the blend is determinedby the relative proportion of Al in the LiF/Al blend. The relativeproportions of LiF and Al in the LiF/Al blend layer are determinedaccording to the desired resistivity of the layer. The desiredresistivity will of course vary according to the required thickness ofthe layer but is basically determined to provide a layer which is not sohigh in resistance that it leads to a significant increase in the drivevoltage (since this will reduce the power efficiency of the device) butis high enough in resistance to ensure that crosstalk between adjacentcolumns is reduced to an insignificant level. The desired resistivitywill therefore depend on several factors such as the number and spacingof the aluminum columns (which will depend on the desired resolution),the voltage at which each column is sequentially driven relative toadjacent columns, and the current density at which the device is to beoperated.

Although a standard back-light LED is operated at a relative low currentdensity of typically 1 mA/cm², the operating current density of adot-matrix display LED will often be higher because, for example in apassive matrix device, the rows are driven sequentially. Typically, thehigher current density will correspond to the unpulsed current density(the current density at which the device would be operated if it were tobe used as a back-light device) multiplied by the number of rows whichare sequentially driven. Therefore, a device having 100 rows willtypically be operated at a current density of 100 mA/cm².

If the layer were to have a thickness of 0.5 microns, the resistivity ofthe LiF/Al blend could be up to 2×10⁴ Ohm.cm without leading to anincrease in drive voltage of greater than 0.1V, and if an increase indrive voltage of up to 1V were to be acceptable, the resistivity of theLiF/Al blend could be up to 2×10⁵ Ohm.cm. If a layer having a thicknessof 0.5 microns and a resistivity of 2×10⁵ Ohm.cm were employed in adevice in which the overlying aluminum layer and ITO anode layer wererespectively patterned to form columns and rows each having a pitch of 1mm, a spacing of 0.5 mm and a length of 50 mm, then the leakage currentto the adjacent columns on either side of the driven column is only 0.5μA (based on the supposition that the driven column is at 10V and theadjacent columns on either side of the driven column are earthed)compared to the current though the device of 250 μA when only a singlepixel is lit.

The embodiment described above also has the advantage that thehigh-resistance layer between the aluminum layer and the light-emissiveorganic layer comprises a material, LiF, which contains a low workfunction element and thus aids the injection of electrons into thelight-emissive polymer, thereby improving the performance of the device.

Another embodiment of the organic light-emissive device according to thepresent invention is shown in FIG. 8. The device shown in FIG. 8 isidentical to that shown in FIG. 7 with respect to the substrate, anodeand organic layers, and identical reference numerals are used to denoteidentical components. The device shown in FIG. 8 differs from the deviceshown in FIG. 7 with respect to the construction of the cathode. Thecathode comprises a layer 414 of lithium fluoride having a thickness ofabout 5 nm. This layer 414 can be deposited by any conventionaldeposition technique but is preferably deposited by a thermalevaporation technique to minimize the damage to the underlying organiclayer. On top of this thin layer 414 of lithium fluoride is deposited alayer 416 of a semiconductor material such as a layer of a physicalblend of lithium fluoride and aluminum to a thickness in the range of0.5 to 1 micron. Next, a layer 412 of aluminum is deposited to athickness of 0.5 microns on top of the layer 416 of lithiumfluoride/aluminum blend to form an ohmic contact. This layer 412 ofaluminum is then patterned using conventional patterning techniques toform a series of parallel columns running in a direction orthogonal tothe series of anode rows. The relatively thick layer 16 of lithiumfluoride/aluminum blend ensures the underlying organic layer isadequately protected from the patterning processes. The resistance ofthe aluminum/lithium fluoride blend layer 416 is such that it does notraise the operating voltage of the device by an intolerable degreewhilst still preventing lateral current leakage (cross-talk) betweenadjacent cathode columns. The provision of a thin layer 414 of lithiumfluoride adjacent the light-emissive organic region enhances theinjection of electrons from the cathode into the light-emissive organicregion.

Although the embodiments shown in FIGS. 7 and 8 show the use of ahigh-resistance electrode layer with a patterned cathode, it couldequally be used together with a patterned anode in the case that an OLEDis produced by first forming a patterned cathode on a glass substrate,depositing one or more layers of organic material on the cathode, andfinally forming an anode on the uppermost layer of organic material. Inthe case of an anode, it is preferred that the electrode layer adjacentthe light-emissive organic region comprises at least one element havinga high work function to enhance the injection of positive chargecarriers (holes) into the light-emissive organic region from the anode.

1. An organic light-emitting device comprising a light-emissive organiclayer interposed between first and second electrodes for injectingcharge carriers into the light-emissive organic layer, at least one ofsaid first and second electrodes comprising a plurality of layersincluding a first electrode layer adjacent the surface of thelight-emissive organic layer remote from the other of the first andsecond electrodes, said first electrode layer having a product ofresistivity and thickness in the range of 0.005 to 10 Ω·cm², andcomprising a material selected from the group consisting of a mixture ofa semiconductor material with an insulator material, a mixture of asemiconductor material with a conductor material and a mixture of aninsulator material with a conductor material.
 2. An organiclight-emitting device according to claim 1, wherein said first electrodelayer comprises a mixture of an insulator material selected from anoxide, a nitride and a fluoride with a conductor material.
 3. An organiclight-emitting device according to claim 1, wherein said first electrodelayer has a thickness of at least 0.5 microns.
 4. An organiclight-emitting device according to claim 1, wherein the first electrodelayer comprises at least one material having a low work function.
 5. Anorganic light-emitting device according to claim 4, wherein the firstelectrode layer comprises LiF/Al.
 6. An organic light-emitting deviceaccording to claim 1, wherein the conductor material is Al or Ag.
 7. Anorganic light-emitting device according to claim 1, further comprising asecond electrode layer on the first electrode layer, the secondelectrode layer comprising a layer of a conductor material.
 8. Anorganic light-emitting device according to claim 2, further comprising asecond electrode layer on the first electrode layer, the secondelectrode layer comprising a layer of a conductor material.
 9. Anorganic light-emitting device according to claim 6, further comprising asecond electrode layer on the first electrode layer, the secondelectrode layer comprising a layer of a conductor material.
 10. Anorganic light-emitting device according to claim 1, wherein theinsulator material is selected from the group consisting of Al₂O₃, SiO₂,LiO, AlN, SiN, LiF and OsE.
 11. An organic light-emitting devicecomprising a light-emissive organic layer interposed between first andsecond electrodes for injecting charge carriers into the light-emissiveorganic layer, at least one of said first and second electrodescomprising a plurality of layers including a thin first electrode layercomprising a low work function material adjacent the surface of thelight-emissive organic layer remote from the other of the first andsecond electrodes, and a second electrode layer adjacent the surface ofthe first electrode layer remote from the light-emissive organic layer,said second electrode layer having a product of resistivity andthickness in the range of 0.005 to 10 Ω·cm², and comprising a materialselected from the group consisting of a semiconductor material, amixture of a semiconductor material with an insulator material, amixture of a semiconductor material with a conductor material and amixture of an insulator material and a conductor material.
 12. Anorganic light-emitting device according to claim 11, further comprisinga third electrode layer on the surface of the second electrode layerremote from the first electrode layer, said third electrode layercomprising a layer of a conductor material.
 13. An organiclight-emitting device according to claim 11, wherein the first electrodelayer comprises a material selected from the group consisting of Ca, Li,Yb, LiF, CsF and LiO.
 14. An organic light-emitting device accordingclaim 11, wherein the thickness of the first electrode layer is in therange of 0.5 nm to 10 nm.
 15. An organic light-emitting device accordingto claim 14, wherein the thickness of the first electrode layer is lessthan 5 nm.
 16. An organic light-emitting device according to claim 11,wherein the insulator material is selected from the group consisting ofAl₂O₃, SiO₂, LiO, AlN, SIN, LiE and OsE.
 17. An organic light-emittingdevice according to claim 11, wherein said second electrode layercomprises a mixture of an insulator material selected from an oxide, anitride and a fluoride with a conductor material.
 18. An organiclight-emitting device according to claim 11, wherein said secondelectrode layer has a thickness of at least 0.5 microns.